Polyurethane coatings have been developed for use as topcoats (i.e., paints) for coating a broad range of objects including vehicles (e.g., wheeled and tracked vehicles, aircraft, ships, etc.), machinery, equipment, buildings, and other structures and articles. Polyurethane coatings have also found use in military applications, for example, as chemical agent resistant coatings (referred to by the acronym “CARC”) for application to military equipment and as military vehicle camouflage coatings.
Polyurethane coatings are presently provided as either single-component or as plural-component coatings. Single-component moisture-curing polyurethane coatings refer generally to a class of coating compositions which are pre-manufactured and which do not require further mixing together of separate coating components before use. Plural-component water-reducible polyurethane coatings refer generally to a class of coating compositions which are provided as separate components in two or more containers. One component includes a polyol and the other component includes an isocyanate. The user must admix the components before use. The polyol and isocyanate react to yield the polyurethane coating.
Single-component moisture-curing polyurethane coatings are generally preferred over plural-component water-reducible coatings because they can be used directly from the container in which the coatings are supplied and, therefore, are easier to use. In contrast, plural-component water-reducible polyurethane coatings are relatively more difficult to use as compared to single-component coatings because of the need to admix the coating components at the point of application and before use.
Single and plural-component polyurethane coatings are most frequently applied to vehicles, machines, and other objects by means of spray application. Single-component moisture-curing polyurethane coatings are typically applied with an air atomizing spray gun directly out of a pressure pot. Plural-component water-reducible polyurethane coatings may be applied with a plural-component proportioning system. Alternatively, the components of the plural-component water-reducible polyurethane coatings may be admixed before use, permitting application with a spray gun. Single and plural-component polyurethane coatings can also be applied with a roller or brush, for example to touch up a coated object.
Single-component moisture-curing polyurethane coatings are typically ready to be sprayed as supplied. Plural-component polyurethane coatings are typically reduced with water during spray application to achieve the desired coating viscosity.
When applying polyurethane coatings to a vehicle, machine, or other object, it is important that the finished-form coating have a consistent and uniform surface free of surface imperfections. “Blisters” are a common type of surface imperfection which should be avoided in the coating applied to the object. A blister or blistering means or refers to formation of dome-shaped projections in a film resulting from local loss of adhesion and lifting of the film from the underlying surface. Blisters can be formed in a polyurethane coating by release of carbon dioxide gas during curing.
Another surface imperfection which should be avoided is referred to as “crazing.” Crazing means or refers to fine lines or very small surface cracks on the film.
Yet another type of surface imperfection is known as “sagging.” Sagging means or refers to accumulations of coating caused by running of the flowable coating shortly after application to the object and before the coating has cured sufficiently to no longer run or flow. The sagging appears as a built up irregularity in the finished-form coating which detracts from the uniformity and consistency of the coating. Sagging is a particular problem with respect to coatings applied to vertical and sloped surfaces.
In addition to aesthetic considerations, a consistent and uniform surface is important to ensure that the coating performs as required. Defects such as blisters, crazing, or sagging can form voids, gaps, and other irregularities in the coating. Voids, gaps, and other irregularities can, in turn, lead to deterioration and failure of the coating. Any deterioration or failure of the coating can result in corrosion, or other damage to the object on which the coating is applied.
Provision of a continuous, uninterrupted coated surface is of particular importance in conjunction with CARC coatings applied to military vehicles and other military objects. This is because CARC coatings must provide a continuous, uninterrupted barrier to penetration of chemical agents to thereby protect the coated object from damage which could be caused by the chemical agent. Further, the CARC coatings must be resistant to harsh decontamination and cleaning agents such as Decontaminant Solution 2 (DS-2) (70% diethylene triamine/28% ethylene glycol monomethyl ether/2% sodium hydroxide) and supertropical bleach (93% calcium hypochlorite/7% sodium hydroxide) which are used to remove chemical agents and clean military vehicles.
Blisters, crazing, sagging, or other surface imperfections are deemed to be defects in the coating. The presence of any of these defects can result in rejection of the coated object by the customer. If the object is rejected, then the defective coating must be removed and the coating re-applied. The process of correcting defects is time consuming, results in increased costs, and is generally to be avoided.
Blistering and crazing are particular problems when applying plural-component water-reducible polyurethane coatings because of carbon dioxide gas formation during curing of the coating. Carbon dioxide gas formation occurs when the plural-component water-reducible polyurethane coating reacts with water in the polyol-containing component or with atmospheric moisture during curing. Carbon dioxide gas may also be formed when the plural-component polyurethane coating reacts with the water which may be used to dilute and reduce the coating viscosity during application to an object. When released from the coating, the carbon dioxide gas can cause formation of blisters and/or crazing in the coating.
Such blistering and crazing is a particular problem when the film thickness exceeds 5 mils (0.005 inch) dry film thickness (DFT) in humid conditions. The blistering and/or crazing occur in thick coating builds because the carbon dioxide cannot escape from the thick film build. Application of the plural-component moisture-curing polyurethane coatings in humid conditions can result in blistering and/or crazing because the water does not readily evaporate from the coating during curing, promoting carbon dioxide formation and the resultant formation of surface imperfections in the cured coating. Because the objects to be painted are of complex configurations, excessive film builds above 5 mils are inevitable. Consequently, the risk of blistering and/or crazing is significant when applying plural-component water-reducible polyurethane coatings to an object.
Plural-component water-reducible polyurethane coatings are relatively slow to cure because these coatings include low levels of catalyst for purposes of maintaining an adequate pot life. Because the plural-component water-reducible polyurethane coatings remain tacky for an extended time after application to the object, such plural-component water-reducible polyurethane coatings must typically be cured in a dust-free booth to prevent dust and other contaminants from contacting and adhering to the coating during curing. The relatively long amount of time to cure the coated object reduces throughput, potentially increasing cost.
Curing of objects in an oven or with an infrared or other energy source is frequently utilized to accelerate curing of the plural-component water-reducible polyurethane coatings. However, use of an oven or other energy source to accelerate curing is energy intensive and imposes further costs on the coating applicator.
Single-component moisture-curing polyurethane coatings avoid or minimize certain of the aforementioned problems associated with plural-component water-reducible polyurethane coatings. As stated, an advantage of single-component moisture-curing polyurethane coatings is that they are used as supplied. Water is not used as a viscosity modifier which limits carbon dioxide gas formation during curing. Water cannot be used to modify the viscosity of single-component moisture-curing polyurethane coatings because use of water causes the single-component moisture-curing polyurethane coatings to immediately gel, rendering the coatings unsuitable for use.
Single-component moisture-curing polyurethane coatings are very good coatings, in part, because they are convenient to use as supplied and do not require viscosity modification as stated previously. However, use of single-component moisture-curing polyurethane coatings without reduction requires a relatively greater amount of the coating from the supplier. This increases the cost to ship and supply the coating to the applicator.
A further shortcoming of existing single and plural-component polyurethane coatings is that these coatings are formulated with solvents including volatile organic compounds (VOCs). There is regulatory pressure to reduce the amount of VOCs in these coatings. A typical single-component moisture-curing polyurethane coating can include about 1 pound of VOC-containing solvents per gallon of coating while a typical plural-component water-reducible polyurethane coating can include about 1.5 pounds of VOC-containing solvents per gallon of coating. It would be desirable to avoid the necessity that a VOC-containing organic solvent be utilized as a reducing agent and it would be further desirable to reduce or eliminate VOC-containing constituents in the polyurethane coatings and to replace those solvents with constituents that are deemed to be more environmentally friendly.
In addition to the foregoing considerations, moisture-curing polyurethane coatings for military applications must comply with stringent technical and performance specifications. Presently, single-component moisture-curing polyurethane coatings must comply with U.S. Department of Defense specification MIL-DTL-53039D and plural-component water-reducible polyurethane coatings must comply with U.S. Department of Defense specification MIL-DTL-64159B. Both specifications presently impose stringent requirements for color and reflectance, hiding power, drying time, specular gloss, infrared reflectance, and other properties and capabilities.
It would be an improvement in the art to provide a water-reducible, single-component, moisture-curing polyurethane coating which would be easy to handle and apply to an object, which would provide a coating free of surface imperfections, which would cure rapidly in ambient conditions, which could be formulated in a more environmentally-friendly manner, and which could be formulated for compliance with current military specifications.